Process for maintaining high oxidation stability in refining of lubricating oils



2,793,982 0N STABILITY 4 Sheets-Sheet 1 May 28, 5 J. w. TIERNEY ET AL PROCESS FOR MAINTAINING HIGH OXIDATI IN REFINING OF LUBRICATING OILS Filed Nov. 19, 1954 ZEFDMZ :9: x EFPDMZ mhsOmsEwFE o IN VEN TORI. JOHN W. TIERNE Y RICHARD E. TAYLOR W ATTORNEY I g o m 04 v o Uni Ins 7 .LHQIBM May 28, 1957 J. w. TIERNEY El AL PROCESS FOR MAINTAINING HIGH OXIDATION STABILITY IN REFINING OF LUBRICATING OILS Filed NOV. 19, 1954 4 Sheets-Sheet 2 N 6E 2580 NEE zochmowm zww xo o m 3 m6 0 v x /O V605 .525 :2: x

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INVENTOR.

JOHN w. TIERNEY. RICHARD E. TALJoR ATTORNEY May 28, 1957 J. w. TIERNEY ET AL 2,793,982

PROCESS FOR MAINTAINING HIGH OXIDATION STABILITY IN REFINING OF LUBRICATING OILS Filed Nov. 19, 1954 4 Sheets-Sheet 4 (D 5 [5 o I x z a: m 5 Z n i 9 2 '2 g g m 9 1- 2 X m m LU Q (n O x 5 LI- 9 0 LL] 2: E E a V 9 m m I Z a 2 1) N 3 L i- 5 w o E; I a 3 w o S m E E O W 0 g E u. 0 u m 5 o 2 I E LL] 3 I 0' 4 0 X m OX O -o x X o x I l l l I l o F 9 ID q: IO N O o o o o 0 o o HIH'IFIS .LH9I3M INVENTOR.

AT TORNE Y United States Patent '0' gllLAsBlLlTY IN REFINING OF LUBRICATING John W. Tierney, Lafayette, Ind., and Richard E. Taylor,

Woodstock, Ill., assignors to The Pure Oil Company,

Chicago, 11]., a corporation of Ohio Application November 19, 1954, Serial No. 469,989

4 Claims. (Cl. 196-2 4) This invention relates to a method of preparing lubricating oil stocks having high viscosity indices with high natural oxidation resistance and pertains more particularly to an adsorption process for lubricating oil refining wherein a particular concentration range of not less than about 0.15 Weight percent and not more than about 0.2 weight percent total sulfur compoundsis attained for high oxidation stability and optimum flow character istics.

It is known in the art that lubricating oil fractions of crude petroleum oils can be refined to enhance the lowtemperature fluidity, oxidation stability, and viscosity indices. Various refining processes have been developed Ice 2,793,982

seem .to be the principal agents responsible for the sta-.

bility of straight mineral lubricating oils. However, although refined mineral lubric-ating oils are generally more stable to oxidation than unrefined oils from the same source, the rate of oxidation of the former seems to speed up with use and the rate of oxidation of the latter tends to slow down with use. This may be explained in part by assuming that the rate of oxidation and sludge formation is governed by the original presence of relatively unstable compounds and is proportional to their concentration; thus, as oxidation proceeds, their concentration decreases and the increase in sludge per unit of time must also decrease. In solvent-refined oils, however, the rate of sludge formation with use increases with time. Therefore, the oxidation must be preceded byrthe formation of substances that oxidize easily, which reaction is accelerated by oxygen, heat and the presence of metals. In unrefined oils the rate of sludge formation decreases with time so the oxidation must be inhibited and the concentration of oxidizable substances must be including sulfuric acid treatment, adsorption, thermal diffusion, solvent extraction, extractive distillation, chemical and clay treatment to accomplish these purposes. These processes inherently remove as a chemical reaction product or solvent extract phase'those non-paraflinic materials which are most unstable and deleterious to the lubricating properties of the oil. Certain of the natural sulfur, oxygen, and nitrogen compounds are also removed during these refining operations. The oxidation properties of lubricating oils are important because in service they are subjected to highly complicated oxidation atmospheres wherein oxidative reactions are catalyzed by metal surfaces and the addends present, under complex variations in temperature, pressure, oxidizing atmosphere and agitation, as the extent of use progresses. As far as the chemical structure of lubricating oils is concerned, little is known except that the aromatic content of higher boiling lubricating oil fractions (450 to 500 C.) of petroleum oils varies from 10 to 40 percent, the naphthenic content from to 80 percent, and the isoparaifinic content from 15 to 75 percent, depending on the nature and source of the oil. Cyclization of the hydrocarbons has an enormous effect on viscosity while branching of the chains in acyclic hydrocarbons has little, if any, effect on viscosity. Likewise, unsaturation has little effect on Viscosity in acyclic hydrocarbons although it may be an influence in cyclic compounds. A finished lubricating oil consisting as it does of these various hydrocarbons and in addition containing from 2 to 20 percent of sulfur compounds, 0.08 to 0.3 percent of nitrogen compounds and some oxygen compounds, represents a complex system for study.

*It has been established, however, that although alkyl naphthenes found in white mineral oil oxidize autocatalytically and are extremely unstable as compared with straight lubricating oils, and alkyl benzenes and alkyl diphenyls are also unstable in spite 'of isolated instances where extremely stable hydrocarbons are found, as among the alkyl naphthalenes, the hydrocarbons in a lubricating oil are of relatively minor importance compared to organic sulfur compounds with respect to resistance to oxidation. Previous investigators have established that the natural acidic or phenolic bodies present in lubricating oils, removable by vacuum distillation from over sodium metal, are not responsibletor the oxidation slowly decreasing. Confirmation of the above and re tracts proved to be intractable and could not be separated or purified 'by ordinary chemical means. In many instances it has been reported that the sulfur-containing fractions did not inhibit the oxidation of the sulfur-free oils. The sulfur compounds themselves must be as complicated as the mixtures of hydrocarbons from which they are extracted. Research onthe oxidation inhibition qualities of synthesized sulfur compounds has indicated that monosulfides are good inhibitors; those sulfur compounds in which the sulfur is a part of a ring are somewhat more active, but mercaptans and disulfides are not as active on an equivalent sulfur basis as the corresponding monosulfides. These differences are partly attributed to the influence of the rate of reaction of the sulfur compounds with peroxides formed or present in oil, and it is generally accepted that the rate of oxygen absorption of oils may be proportional to the peroxide concentration. Those oils which contain natural antioxidants, or agents capable of reducing peroxides, will not build up high peroxide concentrations with consequent high oxidation rates. Sulfur compounds in their reaction with the peroxides are oxidized to deleterious products in the oil. The refiner, therefore, is faced with the dilemma of refining lubricating oils to improve their flow characteristics while still trying to retain a suflicient concentration of natural sulfur compounds to prevent the development of appreciable concentrations of peroxides and concontent, representing all types of sulfur compounds present, of at least about 0.15 percent by weight to about 0.2 percent by weight, there is little loss in oxidation resistance. But if the sulfur content is brought below this critical level, the lubricating oil becomes highly susceptible to oxidation- In other words, a critical correlation has Patented May 28, 1957 been found between total sulfur content and oxidation stability for lubricating base stocks made from Mid- Continent crude oils. By maintaining the sulfur content at a level of at least about 0.15 to 0.2 weight percent, good oxidation stability of the oils is obtained. This critical level or limited range of oxidation inhibiting materials, as measured by the ultimate analysis for total sulfur content, may be produced in the lubricating oil either during refining using any of the known methods, by processing to not lower than 0.15 weight percent sulfur, or subsequent to refining by blending the phase containing the oxidation inhibiting material obtained from such refining back into the highly refined oil to attain this same critical level. Furthermore, it has been found that the concentration of the mixture of sulfur compounds, and not any particular type of natural sulfur compound is primarily responsible for the increased resistance to oxidation found above a level of about 0.15 weight percent.

Since one of the chief causes for lubricating oil deterioration is oxidation and some of the deleterious products associated with oxidation are corrosive acids, sludge, and varnish, the inhibition of which ordinarily calls for various expensive addends, it is advantageous to be able to blend or produce oils having good oxidation stability. Furthermore, the use of oxidation inhibiting addends hinges on the theory that their effectiveness is proportional to their concentrations. Since these addends are of differing eifectiveness and cost, the attainment of a stable level of good oxidation resistance according to the method of this invention not only facilitates the production of highly refined, high VI base stocks with good oxidation resistance but does so economically by minimizing or eliminating the cost for anti-oxidation additives.

Accordingly, it is a primary object of this invention to provide a process for producing lubricating oil base stocks of good oxidation resistance without deleteriously alfecting the. other physical properties and without the need of expensive addends.

Another object of the invention is to provide a refining process wherein lubricating oil fractions and residues from Mid-Continent crude oils may be prepared by reduction of the total sulfur content to a certain critical minimum of about 0.15 weight percent, below which point the oxidation stability rapidly decreases.

Other objects and advantages of the invention will appear as the description thereof proceeds.

Figure 1 is a graph showing the oxygen absorption rate in cc./min. as abscissas plotted against the weight percent of sulfur for neutral stocks.

Figure 2 is a graph showing the oxygen adsorption in cc./ min. as abscissas plotted against the weight percent of sulfur for bright stocks.

Figure 3 shows the relation between sulfur content and modified Sligh test results in mm./min. for neutral oils.

Figure 4 shows the relation between sulfur content and modified Sligh test results in mm/min. for bright stocks.

A Van Zandt crude oil having an API gravity of 33.1 was topped to remove such light fractions as gasoline, naphtha, kerosene and a light lubricating oil, leaving a topped crude having the following characteristics.

TOPPED CRUDE The above topped crude was subjected to vacuum distillation in a tower operated at a flash-zone temperature of 705 F. under absolute pressure of mm. mercury with flash steam in amount of about 0.7 lb./bbl. of charge. A vacuum residue of the following characteristics was produced.

VACUUM RESIDUE Gravity API 12.6 Flash F 575 Vis. SUS 210 F 1300 Color NPA Black Carbon residue (Conradson) percent w. 13.5

The vacuum residue was deasphalted with a 6/1 propane/ oil ratio in a tower having a temperature gradient of to 115 F. from top to bottom and operated at 450 p. s. i. g. to produce a deasphalted oil having the following characteristics.

Gravity 22.4 Vis. SUS 210 F Color NPA +6 Carbon residue (Conradson) percent w. 1.80

The deasphalted oil was next extracted with a 3.5/1,

phenol to oil ratio in a tower having a temperature gradient of 204 F. to 192 F. from top to bottom, dewaxed to a 0 F. pour point, and contacted with 10 lbs./bbl. of clay. By control of the depth of vacuum flashing and solvent extraction, along with proper blending,

there-are produced an intermediate VI Bright Stock, 159 vis. 210 F., VI 93 and a high VI Bright Stock, 152

vis. 210 F., VI 99 which are more completely dc along with proper blending there were produced two new tral stocks hereinafter identified as a 200 vis. intermediate VI neutral and a vis. high VI neutral in the following tables.

Samples of each of these four lubricating oil stocks were percolated through a column of silica gel. This column was 1 /2 inches in diameter and 10 feet long and packed with 20-200 mesh silica gel. The gel Was first wet with hexane to dissipate the high heat of wetting. About 1500 grams of each of the neutral oil stocks described above With 1500 grams of hexane were then passed through the column, followed by 1 /2 gallons of hexane and then 1 /2 gallons of benzene to completely desorb the oil on the gel. The neutral oil and solvent from the bottom of the column were collected in quart container which were then heated in a steam bath using nitrogen stripping to remove the solvent. These neutral cuts were then combined to yield four cuts of approximately 60, 20, 10, and 10 weight percent. The 60 weight percent represented the first, or least strongly adsorbed, portion of the oil. The 20 Weight percent represented that portion of the oil which was fairly strongly adsorbed,

and the 10 weight percent cuts represented the most strongly adsorbed portions.

Similarly, the two bright stocks above described were subjected to percolation through fresh silica gel using higher gel-to-oil ratios than were used with the neutrals. For this operation a 3-inch diameter column 10 feet long was used and only 750 grams of each bright stock was diluted with 2250 grams of hexane. In order to obtain the 1500 grams of oil needed for testing, it was necessary to percolate two batches of each bright stock through the column and then combine the batches. A procedure simithe fractions or cuts obtained by silica gel treatment are resinous, or very heavy compounds, shows a similar trend.

temperature increases from out 1 to cut 4 in each instance as the aromaticity. increases. This could be due to the glven m Table I: 5 fact that the aromatic-type compounds have higher vis- Table 1 PROPERTIES OF VAN BASE OILS AND SILICA GEL FRACTIONS Refrac- Carbon Viscosity, SUS Wt. API Flash, Fire, Color, RI, tivity Sulfur, Neut. Residue, percent Gravity F. F. ASTM 11., Iuter- Wt. N0. Oou- V. I. un-

- cept percent 1948 radson treated 100 F. 130 F. 210 F D11 1111?. VI Neutral.- 29. 4 430 465 +2 1. 4857 1. 0473 0.44 0.10 0. 204 105. 2 46. 3 91 Cut 84- 32.9 1 l. 4728 1. 0440 0. 08 0.05 0. 00 164. 0 87. 3 44. 8 111 57. 6 Out 84-2 30. 7 1+ 1. 4806 1.0458 0. 17 0. 0. 00 181. 7 94. 9 45. 3 99 18. 4 Cut 84-3.. 24. 9 1% 1. 5014 1. 0505 0. 60 0. 05 0. 02 328 149. 4 51. 1 70 14.8 Cut 844.. 11. 6 -|-4% 1. 5577 1. 0647 3. 0. 05, 0.47 1831 512 73. 0 79 9. 2 High VI NeutraL. 31. 3 415 470 l 1. 4782 1. 0450 0. 18 0. 03 e 0.00 177. 1 94. 9 45. 2 103 Cut 85- 33. 6 +0 -1. 4704 1. 0434 0. 05 0. 00 149. 5 85. 6 43. 7 111 e 61.- 6 Cut; 85-2 30. 9 1. 4800 1. 0457 0.11 0.01 184. 7 98. 9 45.4 v 98 14. 1 Gilt 85-3 30.0 1.4842 1. 0474 0. 19 0.00 191.4 100. 9 45. 6 94 11.3 Gut 85-4 26.7 1. 5060 1. 0516 1. 00 0. 03 342 152. 4 50. 8 59 1a. 0 Int. VI Bright H I k Stock 24.7 555. 615 7 1.4988 1.0473 7 0.68, 0.33, 1.23 2,831 1,019 159.1 93

, 0111; 861. 29. 7 +3 1. 4821 1. 0446 0. 14 0.03 0. 17 1, 057 451 100 109 00. 4 Out 86- w 0. 9098 1.5047 1.0498 0.79 "0.03 2.83 3,131 1,092 163.8 90 Y 10.5 Cut 86-3 1 0.9394 1. 5320 1. 0623 '1. 91 3.01 ,5, 660 1, 707, 199. 4 69 14.9 Cut 86-4 1 0. 9320 1. 5258 1. 0594 1. 72 0.48 3.60 1, 502 2, 213 244 75 1 14 2 High VI Bright Stock 26.3 575 640 +6 1.4934 1.0450 0.43 0.16 0.67 2,434 909 152.1 99 0111; 87-1 30. 0 +2 1. 4812 1. 0444 0. 04 0. 02 0. 04 1, 243 517 107. 1 106 62. 3 Cut 87- 24. 9 l. 4974 1. 0465 0. 52 0. 08 0.54 2, 517 941 157. 9 100 12. 6 Cut 87- 19. 5 1. 5210 1. 0540 1. 47 0. 12 2. 50 6, 434 1, 988 237 83 15. 9 Out 87- 19. 2 1. 5203 l. 0524 1. 48 0. 27 3. 54 16, 135 4, 418 417 86 9. 2

. 1 Density at 68 F. (gm.lcc.).

Referring to Table I, it is seen that as would be expected silica gel preferentially adsorbs the more polar compounds, that is, the sulfur compounds, aromatic compounds, and acidic compounds, from the fractions. There is a pronounced increase in sulfur content from cut 1 (least strongly adsorbed) to cut 4 (most strongly adsorbed) in each instance. It is to be observed that for the intermediate V. I. neutral this increase is almost fold. The neutralization number, which is a measure of the acidity, follows a similar pattern. The refractivity intercept, calculated by subtracting onehalf the density at 20 C. in gm./cc. from the refractive index at 20 C. measured with sodium D light (11 is a measure of the aromaticity of the cuts, higher values indicating a more aromatic material. Some typical values of the refractivity intercept for pure hydrocarbons in the lubricating oil boiling range are as follows:

1.0390 Double ring naphthenes 1.0420 Single ring naphthenes 1.0485 Paraifins 1.0620 Single ring aromatics 1.1000 Double ring aromatics From the values of the refractivity intercept shown in Table I it is seen that the separation is mostly between non-aromatics, such as naphthenes, and aromatics in these lubricating oil base stocks. Thus, cut 1, run 84, has a refractivity intercept of 1.0440, which indicates primarily saturates with some aromatics. Cut 4, run 84, on the other hand, has an R. I. of 1.0647, indicating essentially all aromatics. The other cuts show a similar gradation in aromaticity from the least strongly' adsorbed to the most strongly adsorbed portion. Since aromatic compounds have low viscosity indices, the ,V. I. would be expected to decrease from cut 1 to 4 in each instance. The carbon residue, which is an indication of the content of cosities for the same boiling point than the naphthenes, but may also be partially explained by the tendency of silica gel to preferentially adsorb the higher molecular weight material in a homologous series. I

The individual cuts of each of the oils as set forth in Table I were next blended in the following fashion. The first cut, representing about 60 percent of the oil, was used as such and called blend A. The first two cuts blended together, representing about percent of the oil, were called blend B. The first three cuts blended together, representing about percent of the oil, were called blend C, and all four cuts, representinglOO percent of the oil, were called blend D. In this ,method of blending it is to be observed that the last blend, D, in each instance should be identical with the original oil. Likewise, blend C should represent the original oil with about 10 percentof the strongly adsorbed material re moved, and blend B should represent the original oil with about 20 percent of the strongly adsorbed material removed. Similarly, blend A should represent the original oil with about 40 percent of the strongly adsorbed material removed.

The blending was slightly different with the neutral Blend A, run 84, for example, contains all'of cut 1 and enough of cut 2 to make the blend exactly 60 percent of the oil. The other blends made from the neutrals were similarly prepared. This procedure accounts for the nonfractional values under Weight percent of untreated oils in Table II for the neutral oils, with the exception of blend A, run 85. 7

Physical properties of the blends so produced are given in Table II. For the, sake of convenience in following these operations, the adsorption run for the intermediate V. 1. neutral is called No. 84. The run for the high V. I. neutral is called No. 85, the run for the intermediate bright stock, No. 86, and the high V, I. bright stock, No. 87.

Table II PROPERTIES OF BLENDS MADE FROM SILICA GEL FRACTIONS OF VAN BASE OILS Sulfur, Carbon Viscosity, SUS Wt. Gravity, Color, Retrac- Wt. Neut. Residue, Vis- Percent API ASTM R1, in. tivity Per- No. 0011- cosity oi Un- Intercept cent 1948 radson 100 F. 130 F. 210 F. Index trgaitlzed Int. VI Neutral:

Run 84Blend A. 32. 9 1 1. 4729 1. 0441 0.07 0.05 0.00 161. 9 90.4 44.6 110 60.0 Run 84Blend B 32.1- 1 1. 4758 1. 0449 0. 1 1 0.05 0.00 172. 6 92. 6 45.0 106 80. Run 84-Blend C 31. 2 1 1. 4784 1. 0449 0. 0. 05 0.00 180. 9 97. 6 45. 3 101 90.0 Run 84Blend D 29. 4 195+ 1.4819 1. 0435 0. 43 0.05 0.02 202 105. 2 46. 2 93 100. 0 1 (29.4) (+2) (1.4857) (1.0473) (0.44) (0.10) (0.00) (204) (105.2) (46. 3) (91) High VI Neutral:

' R1111 85-Blend A 33. 6 2 +16 1. 4703 1. 0433 0.05 0.05 0.00 149. 2 86; 1 43. 7 111 61. 6 R1111 85-Blend B 32.8 p 4 +12 1. 4728 1. 0438 0.06 0.05 0. 00 157.0 88. 3 44.1 108 80.0 R1111 85Blend 0.- 32. 2 2 +17 1. 4746 1. 0440 0.10 0.05 0.00 161. 7 90.1 44.3 106 90.0

Run 85-Blend D. 31.1 V 3 +1 1. 4776 1. 0438 0. 19 0.00 172.3 93. 3 44. 8 101 100. 0 l (31. 3 (1+) (1.4782) (1.0450) (0.18) (0. 00) (177. 1) (94. 9) (45. 2) (103) Int..VI Bright Stock:

Run 86-Blend A 29. 7 +3 1. 4821 1. 0445 0. 14 0. 03 0. 17 1, 057 451 100. 0 109 60. 4 Run 86-Blend B. 28. 8 +21% 1. 4854 1. 0454 0.23 0.03 0.21 1, 206 503 107. 4 108 70. 9 Run 86-B1end'C 27. 0 +6 1. 4928 1. 0479 0. 51 0. 03 0. 73 1, 435 578 113. 5 102 85. 8

' Run 86-Blend D; 25. 4 +7 1. 4976 1. 0481 0.66 0.11 1.17 1,730 667 123. 2 98 100.0

High VI Bright Stock:

. Run 87-Blend A 30.0 +2 1. 4812 1. 0444 0.04 0.02 0.04 l, 243 517 107. 1 106 62. 3 Run 87.Blend B 29. 4 2 1. 4840 1. 0456 0.13 0. 02 0. 12 1, 387 568 113. 5 105 74. 9 Run 87'Blend C" 28.1 +5 1,4901 1. 0482 0.40 0.02 0. 52 1, 715 678 125.6 101 90.8 Run 87Blend D 26. 6 6 4930 1. 0469 0.46 0.04 0. 61 2, 016 776 136. 8 100 100. 0

. 1 Values in parentheses are properties of untreated oil from Table I.

' I Baybolt Colors.

In Table II, the properties of the untreated 0118 are ture by a sultable bath. As oxygen is absorbed by the 011,

listed in parentheses directly below the properties of blend D and show that in each instance blend D is practically identical with the untreated oil. This shows the accuracy of the experiments, that'the silica gel has brought about no significant chemical change in the oil in the adsorption .process, and that the separation is entirely physical. Obviously, the blends in Table II will show practically the same trends as the individual cuts although the changes will not be as pronounced. The agreement in the trends is very good except for the acid numbers and viscosities of the bright stock. These indicate that a very small amount of acidic, viscous material was so strongly held on the silica gel that it was not desorbcd. Since the material balances in these experiments were over 99 percent, this undesorbed material represents only a very minor amount.

The various blends described in Tables I and II were then tested for oxidation stability using the oxygen absorption test. This test is conducted as follows:

A 30-gm. sample of oil is placed in a glass reaction flask, filling same to about half its height. The flask is fitted with a ground glass cover having a central oxygen delivery tube extending to within about /2 inch of the bottom of the flask. As oxygen passes through the delivery tube it escapes through holes at the bottom end of the tube, mixes with the oil sample to cause a vigorous bubbling action. The oxidation of the oil liberates a certain amount of volatile hydrocarbons and acids as well as other oxidation products which are carried along with the oxygen stream through outlet tubes in the ground-glass cover or stopper leading to a water-cooled condenser. Less volatile materials condense and drop back in the flask. Unreacted oxygen and remaining contaminants leave the condenser top to enter a cold-trap immersed in a mixture of Dry Ice and methanol at a temperature below 100 F. This trap is sufficiently cold to liqu'ify any hydrocarbons and any acids formed during the oxidation. A pump supplies a constant flow of oxygen andthe reaction flask is held at a constant temperaan automatic control system operates to add oxygen from a burette by displacement of the mercury. The amount of oxygen absorbed at any specific time by the oil is indicated 'by the volume of mercury present in a burette. An automatic recorder connected to a leveling device draws a graph of oxygen absorbed.

Oxygen leaving the cold-trap is dried in a Drierite col umn and the carbon dioxide is removed by an Ascarite column. The operation of the cold-trap practically exeludes any water from passing through during operation of the device but since a certain amount of water is likely to be present in the system during the set-up period (before a test is run), it is best removed as soon as possi-' ble. For further details of the construction and operation of such an oxidation apparatus reference may be made to the article entitled, Automatic Tester for Oil Oxidation, Petroleum Processing, October 1953, page 1524. The rate of oxygen absorption in such a circulatory system is unaffected by the volatile and nonvolatile oxidation products formed. This confirms in part the results found by R. W. Dornte and C. V. Ferguson as published in their article entitled, Oil Oxidation, Ind. Engr. Chemistry, 28(7), July, 1936, p. 863 and L. L. Davis et 'al., Oxidation of Petroleum Lubricants, Ind. Engr. Chem istry, 33(3), March, 1941, page 339.

The aforesaid reference is essentially the Davis modification of the Sligh test. In the Sligh test the oxygen is bubbled through the oil at 300 F, with no catalyst present and visual examination of the amount of sludge and varnish formed is made as well as recording the rate of oxygen consumption. In the Davis modification an atmosphere of oxygen is maintained over the oil sample at 347 F. and the time required, usually about 60 minutes for a small pressure change, is noted. These results, for comparison, are corrected to 60 mm. pressure drop assuming a linear relation. The results of oxygen absorption tests are shown in Table III. For convenience, in Table III the sulfur content, neutralization number, and carbon residue of the blends are repeated.

e x 'TbleIIl RESULTS OFOXYGEN ABSORPTION TESTS OF VAN BASE OILS AND BLENDED SILICA GEL FRACTIONS V Carbon Vis.-at 130 F., SUS Sulfur, Neut. Residue, Time, Volume Rate, Sample Tested Wt. No. Con-- Min. Absorbed, cc./Min. Remarks percent 1948 radson p -cc. New Used Percent e Increase Van Int. VI Neutral 0.44 0. 10 p 0.00 2,880 225 0. 089 Van Int. V IzNeutral- '0. 44 0. 10 0. 2, 880 245 0. 085 105. 113. 3 7. 4 Slight Varnish-No Sludge. Van Int; V1 Neutral. 0. 44 0. 0. 00 2, 880 120 0. 042

Run'84-Blend A- 0. 07 0.05 V 0.00 160 500 3.13 86. 2 100. 3 16.4 Slight Varnish-N0 Sludge.

-0. 11 0.05 0.00 000 500 0.833 93. 2 104.4 12.0 No VarnishNo Sludge. 0.15 0.05 0.00 2, 880 410 0.142 96. 8 110. 3 13. 9 Slight Varnish-Slight Sludge. 0. 43 0. 05 0. 02 2, 880 155 0. 047 i i -0. 18 0. 03 v 0. 00 2, 880 185 0. 064 4 0. 18 0. 03 0. 00 880 215 0. 075 94. 7 99. 7 5. 3 Slight Varnish-No Sludge. 0.05 0. 05 0.00 110 500 4. 54 85. 2 96. 0 12. 7 N0 Varnish-No Sludge. Run 85-B1end B 0. 06 0.05 0.00 138 500 3. 62 88. 3 98. 6 11. 7 Run 85B1end C. 0.10 0.05 0.00 1,020 500 0.490 90.4 99. 4 10. 0 Slight VarnishNo Sludge. Run 85BlendD 0. 19 0.00 2, 880 270 0.094 93. 1 99. 6 7.0 No Varnish-No Sludge. Van Int. VI Bright Stk- 0. 68 0. 33 1.23 2, 880 150 0.052 Van Int. VI Bright Stk. 0.68 0.33 1. 23 2,880 200 0.069 1, 019 1, 105 8.4 No Varnish-Slight Sludge. Run 86- 0.14 0.03 0.17 2, 880 300 0.104 '449. 6 615. 3 36. 9 No Varnish-No Sludge.

0. 51 0. 03 0. 73 2, 880 190 0.066 578 872. 2 42. 2 D0. 0.66 0.11 1.17 2,460 480 0.195 667 1,149 72.3 'DO. v .0. 43 0. 16 0. 67 2, 880 135 0.047 920. 9 960. 6 4. 3 N0 Varnish-Slight Sludge. 0. 04 0.02 '0. 04 235 500 2. 13 517. 3 592.0 14. 4 N0 Varnish-No Sludge.

0. 13 0.02 0.12 1, 440 490 0. 340 570.1 629. 3 10.4 D0. 0. 40 0. 02 0. 52 2, 235 .138 0. 062 680. 4 783. 9 15. 2 Run 87Blend D 0. 46 0. 04 0. 61 2, 880 175 0.061 786. 9 923. 6 17. 4

" These results not obtainable for 48 hour test.

For additional confirmation of the results, portions of each of the samples were subjected to the Davis modification of the Sligh oxidation test. The Sligh test as before mentioned measures the rate of decrease in oxygen pressure over a sample of the oil. These results are shown in Table Table IV stocks in Figure 2 and a sharp break in the graph again is observ'ed at avalue of about 0.15 weight percent of sulfur, below which value the oxygen absorption rate in cc./min. is excessive. The additional result's plotted in Figures 3 and 4 show the rate of oxygen pressure decrease in mm./min. with decrease in sulfur content-for RESULTS OF SLIGH OXIDATION TESTS (DAVIS MODIFICATION) Time Cor- Approxi- Rate of Sulphur, Pressure responding mate pressure 011 Wt. Temp, Time, Drop, to 60 mm. of Oxi- Drop Percent F. Min. mm.Hg Pressure dized mm. Drop, Oil, min.

Minutes NPA Int. VI Neutral 0.44 347 307 61 302 6-7 0.199

' Run'84-B1end A 0.07 347 32 .58 33 3 1.82

Run 84-131ei1d B 0.11 347 77 61 76 3 0.789 Run 84-Blend 0-- O. 347 96 60 96 34 0.625 7 Run 84-Blend D 0.43 347 274 62 265 5-6 0. 226 High VI Neutra.1 0.18 347 160 60 160 5 0.375 Run 85-B1end A 0.05 347 17 62 16 2 3.75 Run 85Blend B 0.06 347 65 32 2 1.87 Run 85,Blend C 0.10 347 88 52 102 3+ 0. 588 Run 85-Blend D 0. 19 347 173' 173 4-5 0.347 Int. VI Bright Stock- 0. 68 347 246 60 246 8 0.244 Run 86B1end A- 0.14 347 108 60 108 5 0. 655 Run 86Blend B 0.23 347 156 60 156 5 0. 387 Run 86-B1end O 0. 51 347 168 60 168 8 0. 357 Run 86'.B1end D 0.66 347 92 60 92 8 0. 652 High VI Bright Stock 0.43 347 270 60 270 8 0. 222

- Run 87B1end A 0. 04 347 18 60 18 3-4 3. 83

Run 87-B1end B 0.13 347 114 60 114 8 0.526 R1111 87Blend C 0.40 347 246 60 246 8 0. 244 Run-87-B1end D 0. 46 347 576 60 576 8 0.104

The results of these-oxidation tests as given in Tables III and IV relate only to blends since the oxidation properties of the individual sulfur-rich aromatic fractions were not of primary interest Examination of Tables III and IV shows that the untreated oils and blend D in each instance are the most stable. As sulfur and aromatic compounds-are removed, the oil becomes progressively more unstable. 7 presented in graphic form and reference is therefore made to Figure 1, wherein the volume of oxygen absorption results fonneutrals in cc./min. are plotted against the sulfur content. From this graph, it is apparent that as the sulfur content is reduced from about 0.45 to 0.2 weight percent, the rate of oxygen absorption slowly increase -but at 'a value of about 0.15 weight percent for the sulfur content, the rate of oxygen absorption suddenly and greatly increases and at any values below 0.15 weight percent of sulfur the oxygen absorption rate is excessive. Similar results are shown for the bright neutrals and bright stocks. The results for neutrals are not quite as striking as those for bright stocks. Although the data from the modified Sligh oxidation test are not quite as precise as that from the oxygen absorption test the fact that both neutrals and bright stocks of intermediate and high VI show a break at substantially the same sulfur level is significant.

To establish that the correlation found relating to the critical minimum amount of sulfur necessary to inhibit the oxidation of the lubricating oils is not related to and/or valid only-for the method of treatment used herein, a 15 g. sample of the intermediate V. 1. neutral was treated with 15 g. of finely divided sodium metal at about 450 F. for approximately 5 hours. This sample had a sulfur content of 0.44 weight percent before sodium treatment andafter the treatment exhibited a sulfur content of 0.04 weight percent. This sodium-treated sample showed an oxygen absorptionrate of 2.72 cc. per minute and exhibited a 10.6 percent viscosity increase with no sludge or varnish. It is to be observed that this oxygen absorption rate agrees well with that obtained for the silica gel treatment shown in Figure l, carried to the same sulfur level. Since these two methods of desulfurization are entirely different yet give comparable oxygen absorption rates at the same sulfur level with widely different aromatic contents of the treated samples, it is apparent that the content of sulfur compounds is the determining factor governing the oxygen absorption rate.

The correlation or sharp change in oxidation rate shown in the graphs at about 0.15 weight percent of sulfur could be interpreted to mean that either the type of sulfur compounds in excess of this minimum is not effective for oxidation inhibition, or there is a concentration limit above which additional amounts of the oxidation inhibiting mixture has little further effect. To determine which of these factors most accurately explains the phenomena observed, the following experiments were conducted. A sample of unstable oil (11m 84, cut 1) was blended with portions of blends 2, 3, and 4 from this same intermediate V. 1. neutral oil and the oxygen absorption rates and viscosity increases were observed as follows:

In making the blends shown in Table V, the relative proportions of each cut were selected so that resultant mixture would contain about 0.12 to 0.15 weight percent of sulfur. From these results it is seen that cuts 3 and 4 are at least as effective as cut 2 in reducing the oxidation rate of cut 1, and indicate that the second alternative is the predominant factor in the phenomena. Furthermore, these three points fit quite well in the curve in Figure 1, again independently confirming the phenomena and the accuracy and reliability of the experimental observations.

Another aspect of the invention is the preparation of relatively high V. I. lubricating oil compositions by blending highly refined lubricating oil base stock with sufiicient amount of a concentrate of naturally-occurring, lubricating-oil-range sulfur compounds in the blend to at least about 0.15 percent w. ofsulfur. For this purpose the V. I. of the lubricating oil must be not less than about 90.

Although the invention may be carried out by applying any of the known methods of physical or chemical refining or desulfurization including solvent extraction, distillation, acid treatment, adsorption, thermal diffusion and the like it is particularly adapted to adsorption techniques employing silica gel which is a commercial grade 20-200 mesh silica gel having an apparent density of 40-50 lbs/cu. ft. The refiningmethod used to attain the results observed and claimed herein is only limited to those methods which do not chemically alter the sulfur compounds remaining in the oil. Any combination of two or more physical methods or any combination of two or more chemical methods of refining may be applied. Chemical methods and physical methods may be applied in succession to the oils treated. Since a number of chemical methods, as treatment with aqueous caustic or 2 N sodium aminoethoxide (H2NC2H4ONa) dissolved in anhydrous ethylenediamine, are specific for the removal of certain types of sulfur compounds, as the thiols, or zinc and mercury salts which are specific for cyclic sulfides, the applicationof these methods of refining are not preferred. However, a combination of several chemical methods in succession could be applied 12 to remove the various types of sulfur compounds present to the indicated level.

In addition to the foregoing considerations, the invention is particularly applicable to the mineral lubricating oil fractions obtained from Mid-Continent crude oil, especially the neutral and bright stock fractions having the physical characteristics enumerated.

The term weight percent total sulfur as used herein and in the claims means the percent by weight of all of the sulfur present as represented by any minor amounts of inorganic sulfur compounds present, which is usually insignificant, and the amounts of organic sulfur compounds present based on the lubricating oil.

What is claimed is:

1. The method of preparing mineral lubricating oil stocks exhibiting good resistance to oxidation comprising treating a deasphalted, dewaxed and solvent refined lubricating oil fraction of Mid-Continent origin, containing in excess of 0.2 Weight percent of total sulfur present as naturally-occurring sulfur compounds, with commercial grade 20200 mesh silica gel having an apparent density of about 40-50 lbs/cu. ft. to reduce said sulfur content to a minimum of not less than 0.1 weight percent nor more than 0.2 weight percent based on the lubricating oil.

2. The method of preparing mineral lubricating oil stocks exhibiting good resistance to oxidation comprising, treating a deasphalted, dewaxed, solvent refined bright stock obtained from a residual oil of Mid-Continent origin having an API gravity of about 24.7, a viscosity at 210 F. of about 159 SUS, a neutralization number of about 0.33 and containing about 0.68 weight percent of total sulfur present as naturally-occurring sulfur compounds, with 20200 mesh silica gel at ambient temperatures to remove said sulfur compounds therefrom under conditions such that the total content of sulfur in the form of naturally occurring sulfur compounds is not less than about 0.15 Weight nor more than about 0.2 weight percent based on the bright stock treated.

3. The method of preparing mineral lubricating oil stocks exhibiting good resistance to oxidation comprising, treating a deasphalted, dewaxed, solvent refined bright stock obtained from a residual oil of Mid-Continent origin having an API gravity of about 26.3, a viscosity at 210 F. of about 152.1, a neutralization number of about 0.16, and containing about 0.43 weight percent total sulfur, with 20200 mesh silica gel at ambient temperatures to remove said sulfur compounds therefrom under conditions such that the total content of sulfur in the form of naturally occurring sulfur compounds is not less than about 0.15 weight nor more than about 0.2 weight percent based on the bright stock treated.

4. The method of preparing mineral lubricating oil stocks exhibiting good resistance to' oxidation comprising, treating a deasphalted, dewaxed, solvent refined neutral oil obtained from the lubricating oil fractions of Mid-Continent origin having an API gravity of about 29.4", containing about 0.44 Weight percent total sulfur, having a viscosity at F. of about 204 SUS and a viscosity index of about 91, with 20-200 mesh silica gel at ambient temperatures to remove said sulfur compounds therefrom under conditions such that the total content of sulfur in the form of naturally occurring sulfur compounds is not less than about 0.15 weight nor more than about 0.2 weight percent based on the neutral oil treated.

References Cited in the file of thispatent UNITED STATES PATENTS 2,404,871 Van Ess et al. July 30, 1946 OTHER REFERENCES Mapstone, Australian Chemical Inst. J. and Proc., Vol. 14, pages 61-66, February 1947. 

1. THE METHOD OF PREPARAING MINERAL LUBRICATING OIL STOCKS EXHIBITING GOOD RESISTANCE TO OXIDATION COMPRISING TREATING A DEASPHALTED, DEWAXED AND SOLVENT REFINED LUBRICATING OIL FRACTION OF MID-CONTINENT ORIGIN, CONTAINING IN EXCESS OF 0.2 WEIGHT PERCENT OF TOTAL SULFUR PRESENT AS NATURALLY-OCCURING SULFUR COMPOUNDS, WITH COMMERCIAL GRADE 20-200 MESH SILICA GEL HAVING AN APPARENT DENSITY OF ABOUT 40-50 LBS./CU. FFT. TO REDUCE SAID SULFUR CONTENT TO A MINIMUM OF NOT LESS THAN 0.1 WEIGHT PERCENT NOR MORE THAN 0.2 WEIGHT PERCENT BASED ON THE LUBRICATING OIL. 